Numerical Study of the Spin-wave Velocity in Heisenberg Spin Ladder Model

The columnar-dimer Heisenberg spin ladder model is investigated to calculate the spin-wave velocity v_s under different interchain interaction J_v conditions. The elastic lattice with the elastic force K is introduced into the system which induces spin chains unstable towards the spontaneous dimerization. We obtain the thermodynamic limitations of the singlet-triplet energy gap G and the soliton width W_s when the ladder model is in the Columnar Dimer state by Density-matrix Renormalization-group(DMRG) method and self-consistent method. v s is obtained by measuring the slope of W_s-1/G. The numerical results show that, as J_v increases, G decreases linearly while W_s increases dramatically. According to this, v_s decreases at first and then increases to the infinite and this change becomes sharp when K is bigger. It is noticed that the interchain interaction J_v effects the magnon dispersion.